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Diffstat (limited to 'gcc/invoke.texi')
| -rw-r--r-- | gcc/invoke.texi | 28 |
1 files changed, 27 insertions, 1 deletions
diff --git a/gcc/invoke.texi b/gcc/invoke.texi index a8eb923..9a7ebb1 100644 --- a/gcc/invoke.texi +++ b/gcc/invoke.texi @@ -342,7 +342,7 @@ in the following sections. -mno-wide-multiply -mrtd -malign-double -mreg-alloc=@var{list} -mregparm=@var{num} -malign-jumps=@var{num} -malign-loops=@var{num} --malign-functions=@var{num} +-malign-functions=@var{num} -mpreferred_stack_boundary=@var{num} @emph{HPPA Options} -mbig-switch -mdisable-fpregs -mdisable-indexing @@ -5172,6 +5172,32 @@ If @samp{-malign-functions} is not specified, the default is 2 if optimizing for a 386, and 4 if optimizing for a 486. @end table +@item -mpreferred-stack-boundary=@var{num} +Attempt to keep the stack boundary aligned to a 2 raised to @var{num} +byte boundary. If @samp{-mpreferred-stack-boundary} is not specified, +the default is 4 (16 bytes or 128 bits). + +The stack is required to be aligned on a 4 byte boundary. On Pentium +and PentiumPro, @code{double} and @code{long double} values should be +aligned to an 8 byte boundary (see @samp{-malign-double}) or suffer +significant run time performance penalties. On Pentium III, the +Streaming SIMD Extention (SSE) data type @code{__m128} suffers similar +penalties if it is not 16 byte aligned. + +To ensure proper alignment of this values on the stack, the stack boundary +must be as aligned as that required by any value stored on the stack. +Further, every function must be generated such that it keeps the stack +aligned. Thus calling a function compiled with a higher preferred +stack boundary from a function compiled with a lower preferred stack +boundary will most likely misalign the stack. It is recommended that +libraries that use callbacks always use the default setting. + +This extra alignment does consume extra stack space. Code that is sensitive +to stack space usage, such as embedded systems and operating system kernels, +may want to reduce the preferred alignment to +@samp{-mpreferred-stack-boundary=2}. +@end table + @node HPPA Options @subsection HPPA Options @cindex HPPA Options |